Water jet propulsion unit for use in a jet boat

ABSTRACT

A water jet propulsion unit intended for use in jet boats. The unit has an intake section, a pump section and a nozzle section. In the pump section there are two counter-rotating impellers on concentric counter-rotating shafts calibrated so that any radial flow created in the upstream impeller is converted into axial flow by the downstream impeller. In one embodiment the nozzle section has a throttled outlet to allow for a high mass/low pressure operation while maintaining pump priming. There are no support structures or stators downstream of the intake section in the preferred embodiment.

TECHNICAL FIELD

This invention relates to a water jet propulsion unit primarily for usein jet boats but able to be used in other water craft.

BACKGROUND ART

Water jet propulsion units are of two main kinds, a mixed flow and anaxial flow configuration. A mixed flow unit is one in which the waterenters the impeller parallel to the shaft and is directed radially fromthe shaft and leaves the impeller with radial and axial velocity. Anaxial flow unit is one where the water enters the impeller parallel tothe shaft and also leaves the impeller parallel to the shaft. Thedifferences are more fully explained in the publication "Jet Boating",November 1986, Volume 6, No. 8, page 46.

An example of an axial flow unit may be seen in New Zealand PatentSpecification 123,228 where there is described a motor with twoimpellers, that is a two stage motor having a set of stators between thetwo impellers and another set of stators in the rear nozzle of the jetunit.

In DE 3942672 A1 there is described a mixed flow water jet propulsionunit. In the embodiments described there are two or three impellers inthe pump section whose casing diverges from a narrower cross-sectionalarea at the inlet to a maximum cross-sectional area at the middle andconverges to the minimum cross-sectional area at the outlet. Theimpellers are counter-rotating with respect to each other.

The provision of counter-rotating propellers mounted on concentricshafts is well known from the prior art, for example in U.S. Pat. Nos.4,642,059; 4,832,570; 5,030,149; 5,087,230 and in WO 93/01085. It isdesirable to use a concentric configuration in jet propulsion units soas to minimize obstructions causing turbulent flow within the pumpcasing and also to achieve maximum reliability under the extremeconditions encountered in a water jet propulsion unit.

It is an object of this invention to go some way towards achieving thesedesiderata or at least to offer the public a useful choice.

DISCLOSURE OF THE INVENTION

Accordingly the invention may be said broadly to consist in a water jetpropulsion jet comprising:

an intake section;

a pump section; and

a nozzle section;

the sections being in smooth communication with one another;

a single pair of counter-rotating impellers in said pump section, thedownstream impeller being configured and calibrated to convert anyradial flow created by the upstream impeller into axial flow, saidcounter-rotating impellers being each mounted on separatecounter-rotating drive shafts, said drive shafts extending forwardlyfrom said pump section through said intake section;

drive receiving means outside said intake portion on said drive shafts;

mounting means upstream of said counter-rotating impellers comprisingone or more hydro-dynamic vanes, said counter-rotating shafts beingbearingly mounted in said mounting means;

there being no support structures or stators downstream of said intakesection; and

said nozzle portion having an outlet of cross-sectional area from about0.55 to less than the swept area of the forward of said impellers.

Preferably the outlet cross-sectional area of said nozzle can beadjusted.

This invention may also be said broadly to consist in the parts,elements and features referred to or indicated in the specification ofthe application, individually or collectively, and any or allcombinations of any two or more of said parts, elements or features, andwhere specific integers are mentioned herein which have knownequivalents in the art to which this invention relates, such knownequivalents are deemed to be incorporated herein as if individually setforth.

The invention consists in the foregoing and also envisages constructionsof which the following gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional elevation of a first embodiment of an axialflow pump to be driven by a motor mounted forward of the propulsionunit.

FIG. 2 is a side sectional elevation of a second embodiment axial flowpump to be driven by a motor which is mounted on the outside of thehousing of the intake section of a propulsion unit.

FIG. 3 is a side sectional elevation of a first embodiment of a mixedflow pump in which the motor is also mounted on the housing of theintake seciton of the jet propulsion unit.

FIG. 4 is a side sectional elevation of the rear portion of the intakesection, the pump section and the nozzle section of the embodiment ofFIG. 1 showing a first embodiment of a nozzle throttle.

FIG. 5 is a rear perspective view of the nozzle throttle of FIG. 4.

FIG. 6 is a side sectional elevation of a pump section of the embodimentof FIG. 2 to which a nozzle incorporating an alternative throttlingdevice has been attached.

FIG. 7 is a rear perspective view of the throttling device of the nozzleillustrated in FIG. 6.

FIG. 8 is a side elevational view of a second embodiment of a mixed flowpump having an alternative driving gear.

In each case the drawings have been simplified by omitting the deflectormechanisms associated with the nozzle outlet of the jet propulsion unitfor providing reverse thrust and steerage. Such deflectors are wellknown in the art.

BEST MODE FOR CARRYING OUT THE INVENTION Construction and Operation ofEmbodiment of FIG. 1

The embodiment of FIG. 1 comprises an intake section 10 having anopening 11 flush with the bottom of the hull and covered by a screen(not shown). Immediately downstream of the intake section 10 is a twoimpeller axial flow pump section 12, which comprises a housing 13 andimpellers 14 and 15. The nozzle section 16 is downstream of the pumpsection 12. Bolts 18 secure the nozzle section 16 and the pump section12 to the intake section 10. The pump housing 13 in turn locates thethree vane support 20 containing a water lubricated cutless bearing 21inside the intake section 10. The nozzle section 16 has a frusto-conicalshape having swept internal surfaces which curve into a straight tubularsection at outlet 17.

The impellers 14 and 15 which may each have two or more blades are fixedinto place by keys 23 and 22 and locking nuts 24 and 25 onto separateshafts 27 and 26 respectively. Each shaft is arranged to be driven inthe opposite direction to the other. Each impeller 14 and 15 has itsblades set in opposite orientation to those on the other so that thecancellation effect arising from the impellers 14 and 15 rotating inopposite direction to each other results in axial water flow through thenozzle section 16.

The two driving shafts 26 and 27 pass into a gearbox 28 which is boltedto a flange 30 on the intake section 10. The gearbox 28 is in turndriven by an engine (not shown) which attaches to the gearbox 28 via adrive flange 32 keyed to the inner driving shaft 26.

The inner shaft 26 is supported by bearings 34, 35 and 36, bearings 34and 35 being set inside the outer driving shaft 27, which is in turnsupported by the cutless bearing 21 in pump section 12, and two furtherbearings 38 and 39 in gearbox 28. Within the gearbox 28 are twosprockets 40 and 41 which are linked by a chain 42, shown by brokenlines, and two gears 44 and 45. The first driving sprocket 40 is fixedto the inner driving shaft 26, with power being transmitted to thesecond driving sprocket 41 via the chain 42. The second driving sprocket41 is fixed to a third transmission shaft 46 to which is also fixed oneof the gears 44. This gear 44 meshes with the second gear 45 which isfixed to the outer driving shaft 27. Driving of the input flange 32results in each of the shafts 26 and 27 turning in-opposite directions.Gear 44, 45 and sprocket 40, 41 ratios may be altered but where dieselengines are used to drive the impellers 14 and 15, relative drivingratios are set typically at 1:1 so that both driving shafts 26 and 27turn at the same rate as that of the engine.

Reaction thrust resulting from the impellers 14 and 15 is accepted bythe angular contact bearings 36 and 39. A bearing thrust/support plate47 is fixed inside the gearbox 28 by means of bolts or screws 48 and 49and serves as a means of containment for the rear angular contactbearing 39 supporting the outer driving shaft 27. The outer drivingshaft 27 is fixed into position by two lock-nuts 50 and 51 which lockthe inner bearing hub of the rear angular contact bearing 39 and thegear 45 against a circlip 52.

An additional axial needle roller 54 set inside the inner shaft sprocket40 provides a load bearing surface between the end of the outer shaft 27and the inner shaft sprocket 40, so that the angular contact bearing 36can be pre-loaded when the gearbox lid 56 is screwed or bolted intoplace. An idler sprocket (not shown) serves to take up back-lash in thechain when under driving load.

In order to prevent water entering the space 57 between the two shafts26 and 27 from inside the pump-housing 13, a mechanical seal 58 is setinside the hub of the rear impeller 15. A stainless steel seat 59 forthe mechanical seal 58 is fixed into a groove 60, machined into the backof the retaining nut 25 which locks the upstream impeller 14 into place.

The needle-roller bearing 34 is lubricated by oil which passes throughthe needle-roller bearing 35 from the gearbox 28 into the space 57between the two shafts 26 and 27.

In operation when drive from an engine is engaged with flange 32 itrotates inner shaft 26. This in turn rotates sprocket 40 whose drive istransmitted by chain 42 to sprocket 41 to drive shaft 46. Gear 44 onshaft 46 is rotated and meshes with gear 45 on outer shaft 27 to rotateshaft 27. Thus upstream impeller 14 is rotated on shaft 27 anddownstream impeller is rotated on shaft 26. Water flows through the unitin the direction of arrows A.

Construction and Operation of Embodiment of FIG. 2

The embodiment described with reference to FIG. 2 is an axial flow pumpwhich can be calibrated to operate either as a low pressure/high masspump (operating at up to about 40 psi) or a high pressure/low mass pump(operating at up to about 100 psi). The pump comprises an intake section62 having an opening 63 flush with the bottom of the hull of the boat inwhich it is installed and covered by a screen (not shown). A twoimpeller axial flow pump section 64 comprises a pump-housing 65, whichis a parallel walled tube and impellers 66 and 67. Downstream again is anozzle section 68. Bolts 69 secure the nozzle 68 and pump housing 65 tothe intake section 62. A three vane support 70, containing a waterlubricated cutless bearing 71, is sandwiched between the pump-housing 65and intake housing 62, the support 70 being located centrally in arecess 72 in the intake housing 62.

The upstream intake impeller 66 screws or threads onto the outer drivingshaft 74, seating against a replaceable wear sleeve 76 which in turnlocates against a fixed locating ring 77. A rubber "o" ring 61sandwiched between the impeller hub 78 and the wear sleeve 76 preventswater or contaminants entering the bearing 79 and the space between thetwo shafts 75 and 74. The impeller 66 is of an "axial flow"configuration permitting the incoming water to accelerate along thedriving blades. The accelerated water moves along the inner wall of thepump housing 65 to impinge on the second or downstream impeller 67, alsoof axial flow configuration, which rotates in the opposite direction tothe upstream impeller 66. The effect of this is to straighten the wateras it leaves the downstream impeller blades. The impeller 67 is fixed tothe inner driving shaft 75 by means of a key 80 and a locking nut 81.The downstream impeller hub 82 locates against a wear sleeve 83 and thebearing 79, which in turn bears on a shoulder 84 on the inner shaft 75.The inner shaft 75 is supported/located within the pump-housing 65, bythe bearing 79 located within the hub 78 of the upstream impeller 66 bya snap-ring 85. Both of the driving shafts 74 and 75 are in turnsupported by a cutless bearing 71 inside the three vane support 70. Lipseals 86 pressed into the rear of the upstream impeller hub 78 serve toalso exclude water from the bearing 79.

The two driving shafts 74 and 75 pass into a sprocket/chain transmissionhousing 87 through a mechanical seal 88. The housing 87 is bolted (boltsnot shown) to a flange 89 on the intake section 62 and is furtherattached to a petrol engine 90 which is in turn fixed directly to theintake section 62. In this case a flange 92 fixed to the engine sump 93allows the engine to be bolted with bolts 94 and 95 directly to a flange96 formed as part of the intake section 62. The configuration thus shownin FIG. 2 enables the saving of useful space within small to mediumsized pleasure boats.

A primary drive sprocket 98, fixed to the engine input shaft 99, drivesthe two coupled sprockets 100 and 101 and the drive sprocket 110, fixedto the external drive-shaft 74 via a chain 111 (indicated by a dottedline). Reference should be made to FIG. 2A which describes the means ofproviding counter-rotation of the driving shafts 74 and 75. Thesprockets 100 and 101 are fixed to the same shaft or hub to transmitpower between the chain 111 and the chain 112. The coupled sprocket 101in turn drives the sprocket 113 via the chain 112, the sprocket 113being fixed to the inner drive-shaft 75. Reaction thrust resulting fromthe impellers 66 and 67 is accepted by the angular contact bearing 114,mounted inside the transmission-housing lid 115, with thrust from theouter shaft 74 being transmitted to the bearing 114 via the angularcontact bearing 116, located inside the hub of the sprocket 110.

A mechanical advantage can be provided to the engine by altering thedrive ratio between the primary drive sprocket 98 and the remainingsprockets 100, 101, 110 and 113. It is not intended that the means ofpower transmission previously described should limit the means by whichimpeller rotation can be achieved in that other means are possible whichcould include, for example, the use of gears, belts, chains and/or acombination thereof.

The operation of the embodiment of FIG. 2 within the propulsion unit isidentical to that in FIG. 1. Drive from engine 90 is transmitted throughthe transmission in the manner described in relation to FIG. 2A tocounter-rotate shafts 74 and 75 and their respective impellers 67 and66. Water is propelled through the unit in the direction of arrow A, anyradial flow imparted by impeller 66 being reconverted to axial flow byimpeller 67.

Construction and Operation of Embodiment of FIG. 3

The embodiment described with reference to FIG. 3 can be calibrated tooperate either as a low pressure/high mass pump (operating at the lowestpossible pressure to maintain the intake section 118 and the pumpsection 120 priming at all rotational speeds of the pump to maximizemass flow) or a high pressure/low mass pump (operating at up to about100 psi) comprising an intake section 118 having an opening 119 flushwith the bottom of the hull to the boat in which it is installed andcovered by a screen (not shown). Downstream from the intake section is atwo impeller mixed-flow pump section 120 which comprises a pump-housing121 and impellers 122 and 123. Further downstream is the nozzle sectioncomprising throttling device 124. Bolts (not shown) secure the pumphousing 121 to the intake section 118. A wear ring 125 is fixed to thepump housing 121, locating the three vane support 126 containing a waterlubricated cutless bearing 127 in the intake section 118.

In one embodiment the nozzle throttling device 124 comprises a series ofthin flexible strips 134 (seen best in FIGS. 6 and 7) fixed to acircular rim 128, preferably constructed of stainless steel or otherappropriate material, which fits into a recess 129 in the pump housing121. Fixing screws or bolts (not shown) through flange 135 retain thenozzle section 124 in place to prevent dislodgement by the jet stream.At the end of each strip is a fixed pair of retainers 130 which allowsfor a more or less continuous groove 131 around the end of the nozzlethrottling device 124. The location of the groove 131 is also indicatedby the dotted line in FIG. 7. This groove 131 provides containment for aflexible rubber ring or, alternatively, a coil-spring 132, which whentensioned causes the nozzle opening 133 to contract. Calibration of thetension in the rubber ring or spring 132 is thus a means of providingback-pressure inside the pump housing 121, sufficient to prime the pump.As the pump pressure increases, with increasing flow, the nozzlethrottling device 124 opens and priming is maintained at the lowestpossible pressure throughout the operating range of the pump. Not shownin the drawings is a thin rubber sleeve fitted over the strips 134 (FIG.7) to prevent water loss.

Alternative means of throttling the pump are illustrated in FIGS. 4 and5. FIG. 5 is a perspective view of a nozzle throttling device 136,utilising spring-loaded flaps 137 which can be calibrated to achieve therequired back pressure by altering the tension on each spring 138 by theuse of an externally adjustable screw 140. The flaps 137 are hinged onhinges 141 and are able to move back into a recess 142 in the wall ofthe nozzle throttling device 136 as the flow rate increases. The nozzlethrottling device 136, in this case, can be attached (means ofattachment not shown) downstream of the stern impeller 15 (as seen inFIG. 4) or form part of the nozzle casting or structure.

The present invention is not limited to the means of controlling pumppressure previously described. These means are merely to indicate howthrottling of the pump can be achieved.

The upstream intake impeller 122 screws or threads onto the outerdriving shaft 144, seating against a replaceable wear sleeve 145 whichin turn locates against a fixed locating ring 146. A rubber "o" ring 180sandwiched between the impeller hub 147 and the wear sleeve 145 preventswater or contaminants entering the bearing 148 and the space between thetwo shafts 143 and 144. The impeller 122 is of a "mixed flow"configuration permitting the incoming water to accelerate radially andaxially along the driving blades 149. The accelerated water moves alongthe inner wall of the pump housing 121 to impinge on the downstreamimpeller 123, which rotates in the opposite direction to the upstreamimpeller 122. The effect of this is to straighten the water as it leavesthe downstream impeller blades 150, thereby maximizing reaction thrust.The impeller 123 is fixed to the inner driving shaft 143 by means of akey 151 and a locking nut 152. The impeller hub 153 locates against awear sleeve 154 and the bearing 148, which in turn bears on a shoulder155 on the inner shaft 143. The inner shaft 143 is supported/locatedwithin the pump-housing 121 by the bearing 148 located within the hub147 of the upstream impeller 122 by a snap-ring 157. Both of the drivingshafts 143 and 144 are in turn supported by the cutless bearing 127which is inserted inside the three vane support 126. Lip seals 156pressed into the rear of the upstream impeller hub 147 serve to alsoexclude water from the bearing 148.

The transmission and engine for this embodiment are the same as for theembodiment of FIG. 2.

The two driving shafts 143 and 144 pass into a sprocket/chaintransmission housing 115 through a mechanical seal 88. The housing 115is bolted to a flange 89 on the intake section 118 and is furtherattached to a petrol engine 90, which is in turn fixed directly to theintake section 118. In this case a flange 92 fixed to the engine sump 93allows the engine to be bolted by bolts 94 and 95 directly to a flange96 formed as part of the intake section 118. The configuration thusshown in FIG. 3 enables the saving of useful space within small tomedium sized pleasure boats. A primary drive sprocket 98 fixed to theengine input shaft 99, drives the two coupled sprockets 100 and 101 andthe drive sprocket 110, fixed to the external drive-shaft 144 via achain 111 indicated by a dotted line. Reference should be made to FIG.3A which illustrates the means of providing counter-rotation of thedriving shafts 143 and 144. The sprockets 100 and 101 are fixed to thesame shaft or hub, their purpose being to transmit power between thechain 111 and the chain 112. The coupled sprocket 101 in turn drives thesprocket 113 via the chain 112, the sprocket 113 being fixed to theinner drive-shaft 143. Reaction thrust resulting from the impellers 122and 123 is accepted by the angular contact bearing 114, mounted insidethe transmission-housing lid 87, with thrust from the outer shaft 144being transmitted to the bearing 114 via the angular contact bearing 116located inside the hub of the sprocket 110.

A mechanical advantage can be provided to the engine by altering thedrive ratio between the primary drive sprocket 98 and the remainingsprockets 100, 101, 110 and 113. It is not intended that the means ofpower transmission previously described should limit the means by whichimpeller rotation can be achieved in that other means are possible whichcould include, for example, the use of gears, belts, chains and/or acombination thereof.

FIG. 6 describes a further nozzle throttling device which issubstantially the same as that described in the embodiment of FIG. 3,but which is suitable for an axial flow pump such as that described inrelation to FIG. 2. The outer part (shown in FIG. 7) comprises theflexible strips 134 and attaching ring 135, with the groove 131 andrubber band or spring 139 at the outlet end of the assembly providing ameans of controlling the nozzle outlet area (represented by a dottedline in FIG. 7). The downstream impeller 67 has a cup shaped extension158 attached to its stern end, as a separate fixture, or formed as partof the impeller 67 itself. This extension 158 has its diametercalibrated to the flow rate of the jet emerging from the pump, and alsoacts to prevent air entering the pump in a reverse direction up thecentre of the jet plume, as it emerges from the nozzle throttlingdevice. The impeller extension 158 is thus a functional part of thenozzle throttling device itself.

The extension 159 shown on the end of the impeller hub 153 in FIG. 3also serves the same purpose as that of extension 158 described above.

Operation of Throttling Device

The operation of the propulsion unit in FIG. 1 in conjunction with thealternative nozzle throttling devices in FIGS. 4 and 5 and in FIGS. 6and 7 will now be described. The engine and transmission operation arethe same as described above in relation to FIG. 1. Upstream impeller 14creates a substantially axial flow of water as it passes through theunit in the direction of arrow A. Downstream impeller 15 rotating in theopposite direction reconverts any non-axial flow to an axial flow. Whenthe impellers 14 and 15 are rotating at a low rotational speed, springs138 in the throttle device 136 (FIGS. 4 and 5) urge flaps 137 inwardlyto the limit of their travel. This allows for the build up of sufficientback pressure to prime the propulsion unit at the lowest possible flowthrough pressure. As the rotational speed of the impellers increases,the increasing flow through pressure of water pushes flaps 137 entirelyinto their recesses 142. Thus the water pressure flowing through is ableto be maintained at a substantially constant reduced pressure determinedby the ratings of springs 138 and the ratio of the cross-sectional areaof the outlet of the nozzle device and the swept area of impeller 16.

The throttle unit shown in FIGS. 6 and 7 operates in a similar fashion.Drive is transmitted to impellers 66 an 67 as described in relation toFIG. 2. At lowest rotational speeds and lowest water pressure, rubberband or spring 139 compresses flexible strips 134 together to themaximum extent needed to create the maximum back pressure by minimizingthe nozzle opening area. As the impellers increase their speed, theincreasing flow pressure pushes strips 134 outwardly against the band ofspring 139, maintaining the same sort of equilibrium. In anotherembodiment the spring 139 is tightened mechanically by remote means suchas a bowden cable to enhance priming.

Construction and Operation of Embodiment of FIG. 8

The embodiment illustrated by FIG. 8 generally comprises an intakesection 160, pump-housing section 161 containing impellers 162 and 163,nozzle section 164 and a gearbox 165. Shafts 166 and 167 providecounter-rotation of the impellers 162 and 163. Water from the intake 168is drawn via the intake-housing 160 into the upstream impeller 162 andaccelerated radially and axially around the inner wall of thepump-housing section 161. The accelerated water then impinges on thedownstream impeller 163 which rotates in the opposite direction to theupstream impeller 162. The effect of this is to straighten the water asit enters the pressurized nozzle section 164, thus ensuring that thereaction force or thrust is maximized as the water is ejected from thenozzle section 164. The bowl-shaped pump-housing section 161 is shapedthus so that maximum acceleration of the incoming water from theintake-housing 160 is achieved before it impinges on the second impeller163. The departure from conventional mixed flow pumps, having bothradial and axial flow in the pump-housing section 161, is that aparallel walled section, containing a counter-rotating impeller 163, isfitted downstream of the mixed flow impeller 162. The radial componentof the mixed flow is thereby cancelled before the resulting axial flowenters the nozzle section 164.

A drive flange 169 driven by an engine (not shown) is connected to theinput or impeller driving shaft 167 to which is further attached a bevelgear 170. This bevel gear 170 meshes with a second transmission bevelgear 172 which drives a third bevel gear 173 fixed to the outer impellerdriving shaft 166. To this impeller driving shaft 166 is fixed theupstream impeller 162. The inner impeller driving shaft 167 has thedownstream impeller 163 attached at its nozzle section end 164.

In an alternative embodiment allowing more precise control of gear driveratios and permitting a top mounted engine configuration the engineinput drive may be connected to the vertical shaft of the bevel gear172.

A support 174 attached to the inner wall of the intake-housing 160contains a cutless bearing 175 which supports the outer impeller drivingshaft 166. The outer impeller driving shaft 166 contains a furtherbearing 176 which supports the inner impeller driving shaft 167.Containment bearings for the gears 170, 172 and 173 are not shown.

The operation of the embodiment of FIG. 8 is substantially the same asthat of the embodiment of FIG. 3. The radial component of water createdby impeller 162 is converted to axial thrust by impellers 163.

Calibration of Components

This description broadly outlines a device which maximizes reactionforce by the use of a pair of counter-rotating impellers which in turndrive a pressurized nozzle section.

Further, it is not intended that the scope of the invention be limitedby the means by which the counter-rotation of the impellers is achieved.The driving shafts for the impellers could be driven by a variety ofmeans which could include, for example, the use of chains, sprockets,belts or combinations thereof.

The jet propulsion units herein illustrated can be configured andcalibrated to act either as a high pressure pump able to operate atpressures of up to about 100 psi or a low pressure pump operating atpressures of up to about 40 psi. This section configuration is the moreefficient.

The provision of a throttling device on the nozzle, however, allowsvariation of the cross-sectional area of the nozzle outlet, permittingthe internal pressure of the pump to be minimised still further byallowing the nozzle to open as the impeller speed increases. This meansthat the increasing mass transfer through the pump occurs at the lowestpossible internal pump pressure throughout the operating range of thepump, thus improving the efficiency of the pump.

The operating pressure is controlled by varying the nozzlecross-sectional area, the impeller blade angle or pitch and the impellerspeed. The mode of operation can be determined by the ratio between thenozzle outlet area and the swept area of the upstream impeller:

1. A low pressure fixed nozzle configuration has a 2×30° blade angleimpellers having an outside diameter of 190 mm, a hub diameter of 75 mm,and an axial configuration with a fixed nozzle. The ratio of nozzleoutlet area to swept area of the upstream impeller is about 0.55. Thisratio may be increased by using an adjustable nozzle.

2. A high pressure fixed nozzle configuration has 2×17° blade angleimpellers having an outside diameter of 190 mm, a hub diameter of 75 mmand an axial configuration. The ratio of the nozzle outlet to swept areaof the upstream impeller is around 0.3.

These parameters are not limitative of the invention but areillustrations of how the jet propulsion unit may be calibrated for lowpressure or high pressure operation.

In a mixed flow propulsion unit as is illustrated in FIGS. 3 and 8, thelarger diameter downstream axial flow impeller is calibrated byvariation of the blade angle and peripheral velocity to remove theradial component imposed by the upstream mixed flow impeller.

For low pressure flow propulsion units with larger nozzle outlet areas,throttling is advantageous to ensure that the jet propulsion unit isprimed adequately. As the impellers begin to turn they must supply alarge charge volume of water immediately and some back pressure isrequired for priming.

We claim:
 1. A water jet propulsion unit comprising:an intake section; apump section; and a nozzle section; the sections being in smoothcommunication with one another; a single pair of counter-rotatingimpellers in said pump section, the downstream impeller being configuredand calibrated to convert any radial flow created by the upstreamimpeller into axial flow, said counter-rotating impellers being eachmounted on separate counter-rotating drive shafts, said drive shaftsextending forwardly from said pump section through said intake section;drive receiving means outside said intake portion on said drive shafts;mounting means upstream of said counter-rotating impellers comprisingone or more hydro-dynamic vanes, said counter-rotating shafts beingbearingly mounted in said mounting means; there being no statorsdownstream of said intake section; and said nozzle portion having anoutlet of cross-sectional area from about 0.55 to less than the sweptarea of the forward of said impellers so that the unit is operated in ahigh mass/low pressure mode.
 2. A water jet propulsion unit as claimedin claim 1 in which the outlet end of said intake section and said pumpsection are substantially cylindrical and of the same diameter.
 3. Awater jet propulsion unit as claimed in claim 1 wherein said impellersare axial flow impellers of substantially the same diameter.
 4. Thewater jet propulsion unit as claimed in claim 1 in which the blades onsaid upstream impeller and said downstream impeller are approximately atthe same pitch and the shafts on which they are mounted are calibratedto rotate at the same speed.
 5. A water jet propulsion unit as claimedin claim 1 in which intake section and said pump section aresubstantially cylindrical, said pump section being of greater diameterthan said intake section, the cross-sectional area of said intakesection diverging smoothly to the cross-sectional area of said pumpsection.
 6. A water jet propulsion unit as claimed in claim 5 whereinthe upstream of said impellers is of a mixed flow design and thedownstream of said impellers is of an axial flow design.
 7. A water jetpropulsion unit as claimed in claim 1 in which the nozzle outlet area isadjustable.
 8. A water jet propulsion unit according to claim 7 in whichsaid nozzle outlet area is adjustable by the pressure of water.
 9. Awater jet propulsion unit according to claim 7 wherein said nozzleoutlet area is adjustable manually.
 10. A water jet propulsion unitaccording to claim 1 in which the drive receiving means comprisessprockets.
 11. A water jet propulsion unit according to claim 1 incombination with driving means and transmission means.
 12. A water jetpropulsion unit as claimed in claim 11 in which said transmission meanscomprises chains and sprockets between said driving means and said drivereceiving means.
 13. A water jet propulsion unit according to claim 11in which said transmission means comprises bevel gears between saiddriving means and said drive receiving means.
 14. A water jet propulsionunit according to claim 1 in which said driving means is an enginemounted on said jet propulsion unit.
 15. A water jet propulsion unit asclaimed in claim 11 in which said transmission means is calibrated todrive said impeller shafts at a speed less than the speed of saidengine.
 16. A water jet propulsion unit as claimed in claim 1 which saidnozzle has a fixed outlet area and the ratio of the nozzle outletcross-sectional area to the swept area of said upstream impeller isabout 0.55.
 17. A water jet propulsion unit as claimed in claim 11,wherein said transmission means includes a combination of gears, chainsand sprockets.